Many wireless communications standards require accurate regulation of transmitted power depending not only on the radio link, but also on data rates and signal formats (i.e., coding and modulation methods). As an example, cdma2000 has defined requirements for accurate transmitted (Tx) power control, such as open loop and closed-loop power control in the reverse link. The relative power requirements are also defined for each of the various code channels (e.g., pilot, FCH: fundamental channel; SCH: supplemental channel; DCCH: dedicated control channel; etc.) according to their data rates and signal formats. Further information concerning cdma2000 on transmitted power requirements can be found in section 2.1.2 of IS-2000-2, which is incorporated herein by reference. Nor are such requirements only limited to cdma2000. Many other communications standards also have similar requirements, such as 3GPP UMTS, 3GPP2 1xEVDO. To meet these requirements, accurate calibration and compensation methods must be implemented for such impairments as AGC control characteristic nonlinearity. This nonlinearity is a gain control characteristic which appears as a nonlinearity in the AGC amplifier gain as a function of the control voltage input to the AGC amplifier.
A further source of variation is signal dependent gain variation (signal power level dependent gain variation and signal distribution dependent gain variation), which has heretofore not been compensated for. Signal dependent gain variation is mainly due to the non-purely Class-A nature of an amplifier, or nonlinearity of other devices such as mixers. Most significantly, such signal dependent gain variation is observed in the power amplifiers (PA), which require high power efficiency. In order to achieve high power efficiency, many PAs in the market utilize a self-bias design for the wireless mobile devices, in which the amount of bias is dependent on the input signal strength (instantaneous value or its envelope magnitude or both). Thus, a change of input level varies the gain. Changes in either data rate or signal format result in variation of the signal distribution (e.g., peak to average power ratio). Such changes also typically alter the effective gains in such amplifiers.
It is, therefore, desirable to provide a method and apparatus that compensates for signal dependent gain variations.